Acousto-electric and electro-acoustic transducers using an electret dielectric film (hereinafter also simply referred to as an “electret”) include a condenser microphone and condenser headphones. These transducers use the surface potential of an electret as a polarization voltage and need no polarization power source, such as a DC/DC converter. The transducers are advantageous in that the circuit configuration can he simplified.
Examples of an electret material include FEP (a copolymer of tetrafluoroethylene and hexafluoropropylene) and SiO2. An electret material has the property of being polarized when a high DC voltage is applied and retaining polarization after the voltage is removed.
Negative corona discharge is generally used for polarization processing (electretization processing). Scorotron having a control grid or the like is used to reduce partial variation in surface potential across an electret. An electret varies partially in surface potential due to the surface state, the charge condition, and the like of an electret material.
In the case of a directional condenser microphone, the tension of a diaphragm is designed to be low to secure a bass frequency response. The diaphragm is thus likely to be made to adhere suctionally to a fixed pole. When stability to suctional adhesion is underrun, the diaphragm adheres suctionally to the fixed pole by electrostatic suction force.
In view of this, stability μ to suctional adhesion is set in the design of a microphone so as to prevent suctional adhesion of a diaphragm by the above-described electrostatic suction force from occurring depending on storage environment or use environment. The stability μ to suctional adhesion is represented by the following expression:μ=A×s0×(db3/Eb2)(where A is a proportionality constant including vacuum permittivity, the effective area of a diaphragm, and the distance between the diaphragm and a fixed pole, s0 is the stiffness of the diaphragm, db is the interelectrode distance between the diaphragm and the fixed pole, and Eb is the surface potential (polarization voltage) of the fixed pole).
The stiffness so of the diaphragm and the interelectrode distance db are determined according to a directional frequency response required for a product. It is thus possible to fabricate a directional condenser microphone having a good directional frequency response and high sensitivity by maximizing the polarization voltage Eb within a range which prevents suctional adhesion of the diaphragm to the fixed pole.
When a polarization voltage reaches a suctional adhesion limit in a condenser microphone in which the polarization voltage is applied to a fixed pole by a DC/DC converter or the like, if a diaphragm is a circular film, a central portion thereof is made to adhere suctionally to the fixed pole first. A suctional adhesion area increases toward a peripheral portion with increase in the polarization voltage.
In contrast, if an electret is used, since there is variation in surface potential across the electret, as described above, a portion which is made to adhere suctionally first is not always a central portion. When the electret has a surface potential close to a suctional adhesion limit, variation in surface potential across the electret causes local and partial suctional adhesion.
If the surface potential of an electret is heightened to exceed a suctional adhesion limit, a phenomenon occurs in which the suctional adhesion area spreads gradually. More specifically, suctional adhesion occurs at a portion with a second highest surface potential which is adjacent to a portion as a center that is made to adhere suctionally first. If the first partial suctional adhesion is prevented, the effective surface potential of the electret is kept high, which makes sensitivity higher.
In order to prevent suctional adhesion of a diaphragm to a fixed pole, Patent Document 1 (Japanese Patent Application Publication No. 2003-153395) proposes the idea of making the surface potential of a central portion of a fixed pole lower than that of a peripheral portion. Patent Document 2 (Japanese Patent Application Publication No. 2005-045410) proposes the idea of making a central portion of a fixed pole into an electret-less region.
However, neither Patent Document 1 nor 2 is effective against local and partial suctional adhesion in a portion other than a central portion caused by variation in surface potential across an electret. In Patent Document 1, a central portion and a peripheral portion of a fixed pole are different in surface potential, and polarization processing of an electret is performed in two steps with different corona discharge voltages for the central portion and the peripheral portion of the fixed pole. This leads to low productivity.
Under the circumstances, an object of the present invention is to ensure prevention of partial suctional adhesion of a diaphragm caused by variation in surface potential across an electret dielectric film in a simple way in an electrodynamic acoustic transducer using the surface potential of the electret dielectric film as a polarization voltage.